编程语言c++语言日常笔记
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C++ 字符串拼接效率

C++字符串拼接效率

c++ 提供了 +=、append、stringstream、sprintf 这四种方式来拼接字符串。关于他们之间的性能做一个对比

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class StringEfficiency {
public:
    StringEfficiency() {
        genStrArr();
    }

    void plusTest() {
        std::string res;
        for (int i = 0;i < 10000; i++) {
            res += vec[i];
        }
    }

    void appendTest() {
        std::string res;
        for (int i = 0; i < 100000; i++) {
            res.append(vec[i]);
        }
    }

    void sprintfTest() {
        char res[500000] = {0};
        char* cp = res;
        for (int i = 0; i < 100000; i += 3) {
            sprintf(cp, "%s%s%s", vec[i].c_str(), vec[i+1].c_str(), vec[i+2].c_str());
            cp += 15;
        }
        res[10] = 0;
    }

    void ssTest() {
        std::stringstream ss;
        for (int i = 0; i < 100000; i += 3) {
            ss << vec[i];
            ss << vec[i+1];
            ss << vec[i+2];
        }
    }

private:
    void genStrArr() {
        srand((unsigned)time(nullptr));
        for (int i = 0; i < 100000; i++) {
            std::string str;
            for (int j = 0; j < 5; j++) {
                str.push_back('a' + rand() % 27);
            }
            vec.emplace_back(str);
        }
    }

private:
    std::vector<std::string> vec;
};

int main() {
    auto* se = new StringEfficiency();

    // += 方式
    struct timeval startTime{}, endTime{};
    gettimeofday(&startTime, nullptr);
    for (int i = 0; i < 1000; i++) {
        se->plusTest();
    }
    gettimeofday(&endTime, nullptr);
    long plusTime = (endTime.tv_sec - startTime.tv_sec)*1000000 + (endTime.tv_usec - startTime.tv_usec);
    std::cout << "+= way cost time: " << plusTime << " us" << std::endl;

    // append 方式
    gettimeofday(&startTime, nullptr);
    for (int i = 0; i < 1000; i++) {
        se->appendTest();
    }
    gettimeofday(&endTime, nullptr);
    long appendTime = (endTime.tv_sec - startTime.tv_sec)*1000000 + (endTime.tv_usec - startTime.tv_usec);
    std::cout << "append way cost time: " << appendTime << " us" << std::endl;

    // sprintf 方式
    gettimeofday(&startTime, nullptr);
    for (int i = 0; i < 1000; i++) {
        se->sprintfTest();
    }
    gettimeofday(&endTime, nullptr);
    long sprintfTime = (endTime.tv_sec - startTime.tv_sec)*1000000 + (endTime.tv_usec - startTime.tv_usec);
    std::cout << "sprintf way cost time: " << sprintfTime << " us" << std::endl;

    // stringstream 方式
    gettimeofday(&startTime, nullptr);
    for (int i = 0; i < 1000; i++) {
        se->ssTest();
    }
    gettimeofday(&endTime, nullptr);
    long ssTime = (endTime.tv_sec - startTime.tv_sec)*1000000 + (endTime.tv_usec - startTime.tv_usec);
    std::cout << "stringstream way cost time: " << ssTime << " us" << std::endl;

    delete se;
}

环境：

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c++11
2.6 GHz 六核Intel Core i7
16G

输出如下：

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+= way cost time: 425251 us
append way cost time: 3932957 us
sprintf way cost time: 5121372 us
stringstream way cost time: 7276496 us

因此，我们可以看到 += 方式的效率最高。接下来依次是 append、sprintf、stringstream 方式。